Hexagonal patch printing for orthophoto printers

ABSTRACT

A system is described for providing video signals from left and right stereo photographs which correspond to discrete, homologous areas thereof and which are compensated for parallax and other visual distortion errors. To print a portion of an orthophotograph corresponding to this video signal, a mask defining an hexagonal patch is interposed in the light path between a cathode ray tube for displaying the video signal, and a photographic negative. To construct the entire orthophoto, the negative is mounted on a photo printing device which moves the negative in discrete increments corresponding to selective scanning of corresponding areas in the left and right stero photographs.

United States Patent 1 Hobrough HEXAGONAL PATCH PRINTING FOR ORTHOPHOTOPRINTERS [75] Inventor: Gilbert L. Hobrough, Vancouver,

British Columbia, Canada [73] Assignee: Hobrough Limited, Vancouver,

British Columbia, Canada [22] Filed: Mar. 3, 1972 [21] Appl. No.:231,574

[ Dec.4, 1973 3/1972 Kowalski 250/220 SP 4/1960 Bamet 356/2 [5 7ABSTRACT A system is described for providing video signals from left andright stereo photographs which correspond to discrete, homologous areasthereof and which are compensated for parallax and other visualdistortion errors. To print a portion of an orthophotographcorresponding to this video signal, a mask defining an hexagonal patchis interposed in the light path between a cathode ray tube fordisplaying the video sig nal, and a photographic negative. To constructthe entire orthophoto, the negative is mounted on a photo printingdevice which moves the negative in discrete increments corresponding toselective scanning of corresponding areas in the left and right sterophotographs.

7 Claims, 6 Drawing Figures F/PDM 5a .s'c/l/v 59 GENE/e4 roe VIDEO 5501/7 PUT PATENTED DEC 4 I975 SHEET 1 BF 3 PATENTEDnzc 4 7975' SHEET '2BF 3 GE/YEKA TOE 56 VIDEO 7 OUTPUT PAIENTEDBEc 4 ms 3,777,055

- SHEET 3 OF 3 HEXAGONAL PATCH PRINTING FOR ORTHOPHOTO PRINTERS FIELD OFTHE INVENTION This invention. generally relates to the field ofautomatic orthophoto printers, and more particularly, to an improvementinthe printing of orthophotographs.

BACKGROUND OF THE'INVENTION An improved system which operatesautomatically to provide an orthophotograph for one or more pairs ofstereo aerial photographs is described in my copending applicationentitled AutomaticOrthophoto Printer," U.'S. Ser. No. 760,435,filed'Sept. 18, 1968, now US. Pat No. 3,659,939 which is also assignedto the assignee of the present invention. This system greatly reducesthe time necessary to produce an orthophotograph and includes first andsecond photo-scanning devices which are operated in synchronism toprovide video signals for each of the two photographs making upa stereopair. Homologous areas of the photographs are scanned, with the portionunder consideration being termed the correlation zone. In order toreduce X and Y parallax of the video signalswithin the correlation zone,the system zincludes a correlation network' which operates on the videosignals todetermine the amount of parallax error, and imagetransformation circuitry and raster shaping circuits controlled therebyfor altering the scanning patterns of the two photoscanning devices.Oncethe correlation zones of the two photographs have beenbrought: intoapproximate registryby appropriatev photo positioning devices, thesystem alters the scan pattern of one or both photoscanning devicestoreduce parallax and other errors to zero. One of the resultant videosignals, which represents one image ofthe correlation zone, is suppliedto a cathode ray-tube for imprinting on the photographic negative. Byinvestigating a number of correlation zones, an orthophotograph can beformed.

In order to increase the speed of orthphotograph production, eachcorrelation zone should be as large as possible. The correlation zonesize is determined by the image transformation capability of the system,and the printing method employed. With'respect to the former, thecomplexity of the transformation that is required to eliminate parallaxerror .generallyincreases with an increase in the area of thecorrelation zone. Accordingly, even withthe 50th-order transformationcapability of the system described in the aforementioned copendingpatent application, there is a finite limit upon this area.

The path method of printing has proved itself adaptable to systems suchas described in the aforementioned copending patent application. In thepatch method, an image corresponding to the entire correlation zone isdisplayedon the face of a cathode ray tube and imprinted at one point intime after transformation has been completed.

However, the patch printing method is subject to limitations on the sizeand shape of the correlation zone due to (a) distortions in thereproduced image at the extremities of the CRTs scan, (b) limitations ofthe correlation process, and v(c) limitations in matching patchboundaries mechanically and optically.

It is accordingly an object'of this invention to provide an improvedpatch printing :method for an automatic orthophoto printer.

Another object of this invention is to provide an improvement for anautomatic orthophoto printing system which includes means for producingorthophotographs by the patch printing method in which theaforementioned limitationsv are minimized.

SUMMARY OF THE INVENTION These objects and others are achieved byproviding means defining a hexagonal mask in a light path between theface of a cathode ray tube on which the transformed image is displayedand. the emulsion surface of a negative comprising the desiredorthophoto.

DESCRIPTION OF THE DRAWINGS The above as well as additional advantagesand objects will be more clearly understood with reference to thefollowing description, taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic diagram showing an automatic orthophoto printingsystem;

FIG. 2 is a pictorial view showing the output cathode ray tubeandorthophoto printing mechanism of FIG. 1;

FIG. 3 is a plan view of the hexagonal-masbdefining means shown in FIG.2;

FIG. 4 is another, exploded view of the apparatus illustratedin FIG. 2;

FIG. 5 is a reproduction of an orthophotograph which wasproducedusingthe patch printing method of the prior art; and

FIG. 6 isa representation of the same orthophotograph butproducedusingthe hexagonal patch printing method of this invention.

DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 is a system'diagram of theelements of a preferred embodiment-of the invention shown as anautomated analytical plotter. Diapositives 12A and 128 corresponding tothe stereo aerial photographs are mounted .on the carriages l7 and 18 ofthe scanner and plate transport assemblies 41 and 42. The plates areadapted for movement in the X and Y directions by the X drive motors l9and 20 and the Y drive motors 21 and 22. Light sources 23 and 24illuminate the diapositives and provide light for the scanners 25 and 26(which can be conventional TV pickup units). A scanning'printer 43contains a cathode ray tube 59 and an optical system for printing onsensitive film 43A. A computer 29 which can be any of a number availableon the market solves the basic resection-intersection equations anddelivers stage coordinate commands to the scanners 41 and 42 and toprinter 43, on lines 44X, 44Y, 45X, 45Y, and 46X, 46Y, respectively. Theelectronic viewer 47 enables an operator to observe the images beingscanned. A steering control 48 delivers instructions to the computerduring manual operations. The electronic correlator 49 generates X and Yparallax error signals in response to timing differences betweencorresponding elements of the left and right 'video signals on outputlines 50 and 51 from scanners 25 and 26.

The operation of the system illustrated in FIG. 1 is as follows. First,a sequential program establishes a pair of model coordinates forexamination. Stage coordinate signals are delivered to the printer 43along lines 46X, and 46Y, causing the sensitive film 43A in the printer43 toassume a position corresponding to the selected model coordinate.Second, stage coordinates for the left and right scanners are computedon the basis of an initial or arbitrary terrain height (Z) evaluationfor the center of the scanned area and such coordinate signals aredelivered on lines 44X, 44Y and 45X, 45Y to actuate the transports 41,42, respectively. The correlator operates on the left and right videosignals, on lines 50 and 51 respectively, to determine the X and Yparallax errors of the scanned area. The X parallax error signal fromthe correlator is delivered to the computer along line 52. On theanalysis of this signal, the computer center can order a modification ofthe initial Z value in a direction that will reduce the center Xparallax error to zero. The computer re-evaluates the stage coordinatesof the scanners on the basis of the new Z value and delivers modifiedstage coordinates to the left and right scanners on lines 44X, 44Y and45X, 45Y respectively. The motion of the transport is electronicallycompensated for by the simultaneous cancellation from the memory of thecorrelator of this center-point Z value. In this manner, the range ofthe correlators memory is optimized about the center point of thescanned area. The process of analysis of the X parallax error on line 52and the determination of a new Z coordinate continues iteratively untilthe center-point X error (a zero-order signal) has been reduced to anacceptable level that has made optimum use of the correlators memory. Anaverage Y parallax signal on line 53 is also delivered to the computerand is used during setup and orientation of the model to generate newstage coordinates for the scanners in the Y direction. After orientationthe Y parallax signal should be zero I but during compilation of themodel the computer responds to Y parallax error signals to compensatefor the effects of film shrinkage, optical distortions, etc.

The printer 43 shown in FIG. 1 produces an orthophotograph on sensitivefilm 43A therein. The cathode ray tube in the printer 43 is scannedsynchronously with the scanners 25 and 26 and one of the video signalsfrom the scannersis used to modulate the lightintensity of the scanningspot in the printer. In FIG. 1, the video signal from a left or rightscanner is delivered along line 50 or 51 through the scanner selectorswitch 54 and along line 55 to printer 43. Normally, the left videosignal is selected for printing areas towards the left of the model andthe right video signal is selected for printing areas towards the rightof the model. For this purpose a left-right signal from computer 29 isdelivered to selector switch 54 along line 55A. The computer alsodelivers an inhibit signal on line 56A that is combined in summingcircuit 57A with the video signal from switch 54. The inhibit signalblanks the light output from cathode ray tube 59 to zero except duringthe desired printing period.

The scan generator 56 produces deflection wave forms required forscanning the diapositives and sensitive film. Te scanning pattern orraster is normally square, but as discussed below the raster signals forscanners 25 and 26 are shaped as required for registration. In FIG. 1the deflection wave forms from scan generator 56 are delivered alongline 58 to the printing cathode ray tube 59, and via lines 57 and 59A tothe raster shaper 62 for the left scanner camera, and via lines 57 and60 to the raster shaper 63 for the right scanner camera. Scanningreference signals are delivered via lines 57 and 61 to the correlator49.

The raster shapers 62 and 63 both receive AZ signals from the correlator49 along lines 64 and 65. Raster shapers 62 and 63 also receive signalsfrom computer 29 along lines 66 and 67 respectively.

The raster shapers 62 and 63 modify the square raster wave forms fromthe scan generator delivered on lines 59A and 60 to produce raster waveforms on lines 66A and 67A that produce in scanners 25 and 26 rastersthat are distorted by high order transformations from their normalsquare shape. By this means the left and right stereo images aretransformed in such a manner that the video signals on lines and 51become more similar, and the image in the scanning printer 43 reflectsthe corrections for scale and other distortions arising out ofnonorthogonal conditions when the pictures were taken.

A parallel line or TV scanning pattern can be conveniently used in thesystem of the present invention. The scanning lines run parallel to theX axis of the model so that the X parallax may be detected simply bymeans of timing differences between the left and right video signals.The number of transformation elements, i.e., measurements of timedifferences in the raster, is dictated by the degree of distortion ortransformation that must be applied to accommodate terrainirregularities. The transformation degree that can be achieved is veryhigh, e.g., equivalent to that of a SOth-order system, and is not alimiting fastor to raster size. The number of transformation elements,the size of the raster, and the video information content of the rasterare a function of the precision with which the scanning rasters can becontrolled. Various system parameters can be adjusted to optimizeprecisionlimitations. If a standard scanning raster is used, then themagnification from the film plane to the scanning raster can be selectedto optimize the system resolution. A raster conforming to American TVstandards is convenient for the subject invention.

The American TV (RETMA) raster parameters referred to are as follows:

Line period 63.5 microseconds Field period 1/60 second Single interlacewith two fields per frame, and thus a frame period 1/30 second Aspectratio width/height The ends of the raster can be masked to provide asquare format.

Although the scanners 41, 42 and scanning printer 43 are shown toinclude vidicons and a cathode ray tube, they also may comprise flyingscanners well known in the art.

Present aerial photographs have a resolution of about 20 line pairs/mmor 1,600 resolution elements per square millimeter. A 320 X 320resolution element raster at photo scale would thus be 8 mm square, 8 mmalso being convenient for the subject invention with the optical systemof the scanners being chosen appropriately to match the film to thepickup system.- Since the overlap or model area of the diapositives isapproximately X 220 mm, i.e., 22,000 square mm total, it will be seenthat the scanners will scan only a small portion of the overlap at anyinstant and that 22,000/64 or approximately 350 patches of raster sizemust be investigated to produce a complete orthophotograph of the modelarea.

Owing to the absence of adequate transformation means in prior artsystems for adjusting the scan raster of the scanners, such orthophotoprinters using the patch method of printing have been forced to employ apatch area of less than about 1 square mm in order to avoid visiblediscontinuities between adjacent patches. This small patch size hasseriously restricted the speed of operation of such instruments.However, using the teachings of the present invention for imagetransformation, a patch size of greater than 8 mm square at photo scaleis made possible, and hence a much higher speed of operation can beobtained than has been possible hitherto.

A preferred embodiment of the printer 43 is shown in more detail inFIGS. 2-4. The positioning mechanism of this device is described andclaimed in more detail in my copending application entitled PhotoPositioning Device, Ser. No. 44,305, filed June 8, 1970, now U.S. Pat.No. 3,687,547 which is also assigned to the assignee of the presentinvention.

A frame assembly 70 is adapted to hold the photographic film 43A, whoseemulsion is to be exposed to the orthophotograph. The frame assembly 70rests upon a flat base assembly which includes a plate 71 and isprovided with a suitable surface to provide easy movement of the frame70 acrossthe plate 71. Bearing feet or pads have been used on theunderside of one frame assembly 70 and were found to work well. A pairof geared driving racks 72 and 73 each carry at one end thereof a frameengaging end member 74, 75, respectively. Members 74 and 75 each carry apair of rollers 76 which rotate about horizontal axes and ride on theupper surface of the plate 71. Second pairs of rollers 77 supported forrotation about a vertical axes and each of the members 74 and75 engagethe edges 102, 103 of the frame assembly 70. The outer ends of the racks72, 73 are supported by rollers 78. The edges 102, 103 of the frameassembly 70 are'flat surfaces which intersect at an angle of 90 for theparticular system illustrated herein. The racks 72=and 73 are maintainedperpendicular to the surfaces 102 and 103 and therefore the racks, ifextended, would intersect at an angle of 90.

The frame 70 is maintained in engagement with each of the rollers 77 byany suitable assembly. In the preferred embodiment, the opposite corners79, 80 of the frame 70 have the ends of cables 81 and 82 securedthereto. Cable 82 passes around a guide 83 supported on plate 71, andthen around guide 84 also supported on plate 71. In a similar manner,the cable 81 passes around the guide 84. Two cables extend from guide 84around a guide 85 supported for rotation about a horizontal supportlocated in the plane of the plate 71, through an opening in the plate 71adjacent guide 85, and around a pulley 86 having a weight 87 securedthereto. Through pulley 86, the cables extend through an opening, notillustrated, in plate 71, around an adjustment eccentric 88, and aresecured by their respective ends to clamps 89 attached to the plate 71.The arrangement is such that the weight 87 acting through the cables 81and 82 provides a yielding force to the frame 70 which arches the edges102 and 103 into engagement with the rollers 77 associated with driveracks 72 and 73. Rotation of the eccentric 88 is used for initialadjustment of the cables.

A drive gear 91 secured to the drive shaft of a reversible electricmotor 463 engages the rack 72. In a similar manner, a drive gear 1 12secured to the drive shaft of a second reversible electric motor 46Aengages the drive rack 73. The operation of the reversible electric stepmotors 46A, 46B is interrupted by operation of an associated controlswitch by arms 93, 111 having rollers 94 and 112 engaged with thenon-toothed edge of the associated racks 72 or 73. The arms 93 and 111are pivoted on pins secured to the plate 71. The switches 97 and 113associated with the arms 93 and 111 are adapted to be operated wheneverthe respective arm is moved clockwise. The end members 74 and on theracks 72 and 73 have beveled surfaces, such as surface 92 on member 74,which engage the rollers 94 and 112 whenever the racks reach theirmaximum extent of outward movement. Therefore, the associatedswitch 97or 113 will be actuated to interrupt further drive of the rack.Corresponding beveled surfaces, such as surface 92A on rack 72, on theouter ends-of the racks 72 and 73 serve the same purpose when themaximum extent of inward travel has been reached.

As seen in FIGS. 2 and 4, a lens 101 is aligned with the intersection ofthe lines of travel of racks 72 and 73 so that the center of printingcorresponds to this intersection. More specifically, this intersectionoccurs at the intersection of the pitch lines of the drive racks 72 and73.

The film 43A can be held in position in the frame 70 by various means.For purposes of illustration, the film 43A is secured to a backing plate70A. A corner stay 98 is shown as being in engagement with one corner ofthe plate 70A with springs 99, 100 urging the corner stay 98 and plate70A towards the opposite corner of the frame 70.

Theorthphotograph is constructed by first establishing a pair of modelcoordinates within the computer 29, then delivering stage coordinatesignals via lines 46X, 46Y to the motors 46B, 46A of the printer 43, soas to cause the frame 70, and therefore the film 43A, to assume aposition with respect to the optical center lens 101 corresponding tothe selected model coordinates. At this point in time, the cathode raytube 59. is blanked by the inhibit signal appearing on line 56A.Subsequently, the computer causes the scanners 25 and 26 to reduce thecenter-point X parallax to an acceptable level and causes the scannertransports to reduce the Y parallax to zero by movements in response tostage coordinate signals delivered on lines 44X, 44Y, 45X, 45Y.Thereafter, the higher order X parallax is reduced to zero in responseto signals from the correlator 49, and subsequent operation of theraster shapers 62 and 63.

When X parallax and other errors are eliminated, the inhibit signal isremovedfrom line 56A and the video output of either scanner 25 or 26, asdetermined by the left-right signal on line 55A, is coupled via line 55to the cathode ray tube 59. Accordingly, the corrected video signalappears as an image 104 on the face of the scanner 59. This image 104will be coupled through the lens 101 to expose a portion of the film43A. After exposure iscomplete, the computer 29 selects new modelcoordinates and delivers appropriate stage coordinate signals via lines46X, 46Y, so that the printer 43 assumes a new position. Thereafter, thecycle is repeated.

When the entire overlap area of the stereo photographs has been treated,the orthophotograph appears, as in FIG. 5, to comprise a plurality ofadjacent, square patches 110.

If each of the patches reproduces its corresponding correlation zonewith absolute fidelity, and if the patches 110 are precisely alignedwith each other, the boundaries between patches would be practicallyindistinguishable. However, errors in alignment do occur,

despite the use of a precise and accurate plotting device such asillustrated in FIG. 2. The aforementioned difficulties of obtaining auniform, non-distorted trace on a cathode ray tube contribute todistortions in position as well as light intensity at the extremities ofeach patch. Accordingly, the boundaries between adjacent patches oftenappear as a plurality of vertical and horizontal lines, 111, 112.

Although in most cases these lines are but mildly distracting, theybecome of significance when they coincide with an important feature ofthe orthophotograph. For example, the vertical line 111A in FIG. tendsto obscure a portion of a runway 113 depicted therein. This distortionmay materially lessen the value of the orthophotograph in cartographicapplications, wherein very accurate detail is required.

Accordingly, the method of the invention allows construction of anorthophotograph by a hexagonal patch.

With reference now back to FIGS. 2-4, a plate 106 includes a pluralityof apertures 107 for receiving a corresponding plurality of pins 105which are secured in plate 71. Plate 106 also defines acentrally-located, hexagonal aperture 108 whose area corresponds to adesired area of each patch making up a composite orthophotograph. Theplate 106 is generally positioned so as to align the center of aperture108 with the center of optics 101. Therefore, the hex patch which isprinted is centered on the image 104 reproduced by CRT 59.

To construct the orthophotograph, the model coordinates are selected bythe computer 29 so that the frame 70 is first moved in uniformincrements along the Y axis until the Y-dimension of the orthophotographhas been traversed. Then, the X model coordinate is moved by anappropriate increment, and the Y model coordinate is moved by anincrement that is out of phase with the adjacent Y coordinate in adirection opposite to that previously traversed. Printing thereafterproceeds in uniform increments along the Y axis in the oppositedirection until the Y-dimension is again traversed, at which time thecycle is repeated. The X and Y increments should be chosen in view ofthe dimensions of hexagonal aperture 8 so that adjacent patches arejuxtaposed.

A resultant composite orthophotograph is seen to comprise, in FIG. 6, aplurality of adjacent, hexagonal patches 114. Although problems of patchmisalignment may still occur, the tracing errors occurring at the edgesof the image 104 are reduced due to the masking effect of plate 106.More important, misalignment and tracing error are much less apparent inthe orthophotograph of FIG. 6, in that there are no continuous straightlines extending across the photograph for the observers eye to follow.In addition, important features of the overlap area are not obliterated,as might be in the case using a rectangular patch such as in FIG. 5.

Because inaccuracies may yet arise from CRT tracing error and from thedifficulty of effecting a complete transformation over a given overlaparea, the size of the hexagonal aperture 108 may be varied to allow moreprecision and accuracy in the composite orthophotograph. Accordingly, anumber of plates 106 may be provided, each having a different-sizedhexagonal aperture 108. As the aperture size is changed, the programmedmodel coordinates in the computer 29 must also be changed to assureaccurate spacing of the patches 114.

While this invention has been described with reference to a preferredembodiment thereof, it is to be clearly understood by those skilled inthe art that the invention is not limited thereto, but rather isintended to be bounded only by the limits of the appended claims.

What is claimed is:

1. A method for printing an orthophotograph from the overlap area of apair of stero photographs onto a sensitized film, comprising the stepsof scanning a patch on each of said pair of stereo photographs about apoint established by a predetermined set of model coordinates, eachpatch comprising a portion of said overlap area representing homologousareas of said pair of stereo photographs, providing an image from saidscanning which corresponds to said homologous areas and which iscompensated for parallax and other distortion errors, masking the edgesof said image to provide a hexagonal image, and exposing a portion ofthe sensitized film to said hexagonal image, further comprising thesteps of repeatedly modifying said set of model coordinates bypredetermined increments, and correspondingly adjusting said areas ofscanning and the relative positions of said sensitized film and saidhexagonal image so that adjacent patches of said pair of stereophotographs are successfully investigated and printed.

2. In an orthophoto printing system comprising in combination a photopositioning means for holding first and second photographs making up astereo pair, a scanner means including first and second scanners alignedwith said photographs and each including raster signal means forcontrolling scanning of a selected patch on each photograph, each patchbeing an area of a photograph covered in one complete scan cycle withthe scan pattern for each scan cycle being a plurality of adjacentparallel scan lines, a printing means including a scanning display meansand means holding a segment of sensitized film in alignment with saidscanning display means, said scanning display means including meansproviding an image corresponding to the undisturbed scan pattern of oneof said vidicons for the exposure of a patch of film made up of aplurality of adjacent lines, means connecting said scanning displaymeans with said scanner means to render said scanning display meansunder the control of signal information derived from said scanner means,signal correlating means coupled with said scanner means and operativeto derive error signals proportional to the timing differences betweenhomologous components of the video signals provided by said first andsecond scanners, means connecting said correlating means to said rastersignal means in response to said error signals to alter the scan patternfor at least one of said first and second scanners, and signal memorymeans connected between said correlating means and said raster signalmeans for storing the error signals derived from a complete area beingscanned by said scanner means, said signal storage means providingoutput signals to control the alteration of the raster of at least saidone of said scanners during the operation of said scanning display meanswith the image for said scanning display means remaining the same as anundisturbed scan pattern of one of said scanners, an improvement in saidprinting means comprising means defining a hexagonal aperture, saidmeans being interposed between said scanning display means and saidsensitized film for masking the edges of said image of said scanningdisplay means, to reproduce on said sensitized film a hexagonal patchcorresponding to a portion of said undisturbed scan pattern of one ofsaid vidicons.

3. The improvement as recited in claim 2, further comprising meansrepeatedly moving said photographs and said sensitized film with respectto said scanner means and said scanning display means, respectively, inpredetermined increments dependent in part on the size of said hexagonalaperture, said movements being effected after the completion of eachraster alteration caused by said signal storage means.

4. The improvement as recited in claim 3, further comprising meansblanking said scanning display means until the completion of each rasteralteration caused by said signal storage means.

5. An automatic orthophoto printing system, comprising means forproviding a video signal from a pair of stereo photographs whichcorresponds to patches thereof representing homologous areas in aportion of the total area of said stereo photographs and which iscompensated for parallax and other visual distortion errors, a scanningdisplay means for displaying an image corresponding to said videosignal, means masking said image to produce a hexagonal image, meansexposing a sensitized film to said hexagonal image, a photo positioningmeans for adjusting the relative position of said hexagonal imagedisplayed on said scanning display means in said sensitized film, andfurther including means for repeatedly moving the centers of saidpatches and the relative positions of said sensitized film and hexagonalimage in discrete increments to construct an orthophotograph.

6. A system as recited in claim 5, wherein said scanning display meanscomprises a cathode ray tube.

7. A system as recited in claim 5, wherein said scanning display meanscomprises a flying spot scanner.

1. A method for printing an orthophotograph from the overlap area of apair of stero photographs onto a sensitized film, comprising the stepsof scanning a patch on each of said pair of stereo photographs about apoint established by a predetermined set of model coordinates, eachpatch comprising a portion of said overlap area representing homologousareas of said pair of stereo photographs, providing an image from saidscanning which corresponds to said homologous areas and which iscompensated for parallax and other distortion errors, masking the edgesof said image to provide a hexagonal image, and exposing a portion ofthe sensitized film to said hexagonal image, further comprising thesteps of repeatedly modifying said set of model coordinates bypredetermined increments, and correspondingly adjusting said areas ofscanning and the relative positions of said sensitized film and saidhexagonal image so that adjacent patches of said pair of stereophotographs are successfully investigated and printed.
 2. In anorthophoto printing system comprising in combination a photo positioningmeans for holding first and second photographs making up a stereo pair,a scanner means including first and second scanners aligned with saidphotographs and each including raster signal means for controllingscanning of a selected patch on each photograph, each patch being anarea of a photograph covered in one complete scan cycle with the scanpattern for each scan cycle being a plurality of adjacent parallel scanlines, a printing means including a scanning display means and meansholding a segment of sensitized film in alignment with said scanningdisplay means, said scanning display means including means providing animage corresponding to the undisturbed scan pattern of one of saidvidicons for the exposure of a patch of film made up of a plurality ofadjacent lines, means connecting said scanning display means with saidscanner means to render said scanning display means under the control ofsignal information derived from said scanner means, signal correlatingmeans coupled with said scanner means and operative to derive errorsignals proportional to the timing differences between homologouscomponents of the video signals provided by said first and secondscanners, means connecting said correlating means to said raster signalmeans in response to said error signals to alter the scan pattern for atleast one of said first and second scanners, and signal memory meansconnected between said correlating means and said raster signal meansfor storing the error signals derived from a complete area being scannedby said scanner means, said signal storage means providing outputsignals to control the alteration of the raster of at least said one ofsaid scanners during the operation of said scanning display means withthe image for said scanning display means remaining the same as anundisturbed scan pattern of one of said scanners, an improvement in saidprinting means comprising means defining a hexagonal aperture, saidmeans being interposed between said scanning display means and saidsensitized film for masking the edges of said image of said scanningdisplay means, to reproduce on said sensitized film a hexagonal patchcorresponding to a portion of said undisturbed scan pattern of one ofsaid vidicons.
 3. The improvement as recited in claim 2, furthercomprising means repeatedly moving said photographs and said sensitizedfilm with respect to said scanner means and said scanning display means,respectively, in predetermined increments dependent in part on the sizeof said hexagonal aperture, said movements being effected after thecompletion of each raster alteration caused by said signal storagemeans.
 4. The improvement as recited in claim 3, further comprisingmeans blanking said scanning display means until the completion of eachraster alteration caused by said signal storage means.
 5. An automaticorthophoto printing system, comprising means for providing a videosignal from a pair of stereo photographs which corresponds to patchesthereof representing homologous areas in a portion of the total area ofsaid stereo photographs and which is compensated for parallax and othervisual distortion errors, a scanning display means for displaying animage corresponding to said video signal, means masking said image toproduce a hexagonal image, means exposing a sensitized film to saidhexagonal image, a photo positioning means for adjusting the relativeposition of said hexagonal image displayed on said scanning displaymeans in said sensitized film, and further including means forrepeatedly moving the centers of said patches and the relative positionsof said sensitized film and hexagonal image in discrete increments toconstruct an orthophotograph.
 6. A system as recited in claim 5, whereinsaid scanning display means comprises a cathode ray tube.
 7. A system asrecited in claim 5, wherein said scanning display means comprises aflying spot scanner.